How to work with parameters?
First set the Units in the Scene properties to Metric or Imperial. If you are using Imperial, then checking Separate Units can help make it more natural.

For 2.8x 

For 2.7x 

When entering in units in Object mode using the Dimensions, watch the scale. You have to apply the scale ( ctrl A > Scale ) before the same measurements will show in edit mode. When the scale of the object is applied (all three field set to 1)  then dimensions will work as excepted in edit mode.